A variety of communication apparatuses have been proposed that control a half-value angle and a directivity angle of an antenna; refer to Japanese Patent Application Laid Open No. 2002-151936 and Japanese Patent No. 3,562,420 for particulars.
Following is a description of a communication system that employs such a communication apparatus, with reference to FIG. 4. Reference numeral 101 is an initiator node, as of a transmitter communication apparatus. Reference numeral 102 is a responder node, which is a destination communication apparatus. The initiator node 101 has a half-value angle of a transmit antenna set to φI, and a directivity angle thereof set to ΘI. A coverage area of the transmit antenna of the initiator node 101 is denoted by a reference numeral 401. The responder node 102 has a half-value angle of a receive antenna set to φR, and a directivity angle thereof set to ΘR. A coverage area of the receive antenna of the responder node 101 is denoted by a reference numeral 402. As depicted in FIG. 4, the initiator node 101 is compared with the responder node 102, a large half-value angle, i.e., φI>>φR, is obtained thereby, and, as a consequence thereof, the directivity angle thereof is determined to be approximately uniform therewith. Conversely, the responder node 102 obtains an advantage of increasing a degree of freedom of the directivity angle ΘR, and of impeding an interference from another communication apparatus, by narrowing the half-value angle.
As depicted in FIG. 5, however, if a direction of the responder node 102 changes while the half-value angle and the directivity angle thereof remain narrowed, it will become necessary to reacquire the initiator node 101 in order to sustain a communication therebetween. If the responder node 102 possesses an active mode and a sleep mode, then, if the change in the direction thereof occurs while the responder node 102 is in the sleep mode, the responder node 102 will become incapable of receiving a transmission signal from the initiator node 101.
Put another way, if the direction of the responder node 102 changes when the responder node 102 is in the sleep mode while the half-value angle and the directivity angle thereof is narrowed, the responder node 102 will become incapable of receiving a wake-up command from the initiator node 101. Accordingly, the responder node 102 will be unable to resume receiving a data therefrom.